Unit 4:  The genetic revolution                                                       1   2   3   4  

3.2. Stem cells

Stem cells are undifferentiated cells which can divide endlessly producing new stem cells and differentiate in one or more cellular types, such as muscle cells, blood cells or hepatic cells.

a) Types of stem cells

• Embryonic stem cells

When a spermatozoid fertilises an ovum a zygote is formed, a single totipotent cell able to produce by successive cell divisions (mitosis) all the cellular types of the new individual and those of the embryonic annexes (e.g. placenta)

The resultant cells of the first divisions of the zygote are also totipotent. But as new cells each time more specialised are formed, the range of cellular types that they can produce is reducing progressively.

A few days after fertilisation, the first specialization starts. The early embryo is integrated by a series of cells that form a hollow sphere called blastocyst. In it is possible distinguish two cellular types:

- Surface cells (they will produce the placenta).

- Interior cells (they will produce the embryo). These are the embryonic stem cells.

The embryonic stem cells are pluripotent cells, because although they cannot by themselves give origin to an organism (they need the placenta), are the origin of all the cellular types of the adult organism.

• Adult stem cells

These cells are located in the adult organism tissues, such as blood or skin. Their function is to replace the cells that die in order to maintain the tissue. They are multipotent cells able to generate many cellular types, but not all of them.

• Other stem cells

- Fetal stem cells (from aborted embryos because of medical or natural causes)

- Umbilical cord cells (adult cells with plasticity similar to the embryonic cells)

- Germinal embryonic cells (cells producers of gametes which can give all the cellular types of the adult in appropriate conditions of growth)

b) Applications of stem cells

• Testing toxins and new medicines. Today, cancer line cells are been used to analyse antitumor drugs.

• Cellular therapies and transplants. There are regenerative therapies based in the use of adult stem cells to repair organs and damage tissues. It is use in leukaemia (the bone marrow transplant is the best known) and other kinds of cancer and also in cardiac muscle repair. The discovery of stem cells in the adult brain able to produce different kinds of nerve cells and that can regenerate neurons, opens big possibilities to the development of new regenerative therapies. However for many scientists the true regenerative medicine will start when embryonic stem cells can be use, especially those of the own patient. Their clinic application could solve in the future two big problems associated to transplants: lack of donors and the rejection, due to the patient could receive a self-transplant.

3.11. What is the difference between adult stem cells and embryonic stem cells?